The present invention relates generally to color imaging assemblies which employ multilayered dichroic composites for generating spatially separated, color component images of an object on an image plane. The present invention also relates generally to color combiners which employ multilayered dichroic composites for combining separate beams of light of different spectral ranges into a single combined beam. The invention relates particularly to a beam splitter/combiner with a path length compensator which is used in association with dichroic composites.
A number of different dichroic composites are described in U.S. Pat. No. 4,709,144 for COLOR IMAGER UTILIZING NOVEL TRICHROMATIC BEAMSPLITTER AND PHOTOSENSOR of Vincent, and U.S. Pat. No. 4,870,268 for COLOR COMBINER AND SEPARATOR AND IMPLEMENTATIONS of Vincent et al., which are hereby incorporated by reference for all that is disclosed therein. An optical scanner which employs a beam splitter is described in U.S. Pat. No. 4,926,041 for OPTICAL SCANNER of Boyd, which is hereby incorporated by reference for all that is disclosed therein. A beam splitter/combiner with a path length compensator is described in U.S. Pat. No. 5,040,872 for BEAM SPLITTER/COMBINER WITH PATH LENGTH COMPENSATOR of Steinle, which is hereby incorporated by reference for all that is disclosed therein.
The phrase "beam of light" as used herein is broadly defined as any narrow shaft of light having light rays traveling in the same general direction. The phrase "beam of light" therefore includes the light which emanates from an object and passes through the aperture of an imaging lens as well as the converging cone of light which emerges from the lens and is focused on an image plane.
A prior art color imaging assembly 10, as shown in FIG. 1, forms spatially separated color component images, e.g. blue, green and red images 12, 14, 16 of an object 20, such as the scan line of an optical scanner, on a unitary image plane PP. The color imaging assembly 10 may include an imaging lens assembly, shown schematically at 22, adapted for receiving a polychromatic imaging light beam 24, having a central beam axis CC, from the object 20 for imaging the object on the unitary image plane PP. A dichroic beam splitter 26 is disposed obliquely in the path of the imaging light beam 24 for splitting the imaging light beam 24 into a plurality of parallel, spatially separated, color component beams 40, 42, 44 having parallel central optical axes BB, GG, RR. The color component beams 40, 42, 44 may be received by a photosensor assembly 50, which is positioned in alignment with the unitary image plane PP, and which is adapted to transmit sensor signals to a suitable data processing and data storage unit (not shown). The photosensor assembly 50 may be comprised of a plurality of coplanar linear photosensor arrays 52, 54, 56 which are aligned with each of the plurality of component beams 40, 42, 44. Each of the linear photosensor arrays 52, 54, 56 is adapted to transmit a signal representative of a color component image 12, 14, 16 of the object 20 which is focused on the unitary image plane PP.
A path length compensator 60 is disposed between the beam splitter 26 and the unitary image plane PP for refractively compensating for differences in optical path lengths of the plurality of color component beams 40, 42, 44 such that the color component image 12, 14, 16 provided by each of the color component beams 40, 42, 44 is focused on the unitary image plane PP.
The beam splitter 26 may be of the type described in U.S. Pat. Nos. 4,709,144 and 4,870,268 incorporated by reference above. Specifically, the beam splitter 26 shown in FIG. 1 consists of a precisely ground and polished glass bar or plate comprising multiple layers of selected multilayer dielectric interference optical filter coatings 30, 32, 34 (hereinafter "dichroic layer(s)"). Due to the complexity of the coatings 30, 32, 34, the beam splitter 26 is usually comprised of a glass substrate such as BK-7, upon which the coatings 30, 32, 34 are applied. Each of the coatings 30, 32, 34 typically have twenty to thirty unique layers.
The beam splitter 26 shown in FIG. 1 comprises a first dichroic layer 30 which reflects light in the blue spectral band, a second dichroic layer 32 which reflects light in the green spectral band, and a third dichroic layer 34 which reflects light in the red spectral band.
The path length compensator 60 may be of the type described in U.S. Pat. No. 5,040,872 incorporated by reference above. Specifically, the path length compensator 60 shown in FIG. 1 is comprised of a prism 61 with an angled, planar surface 62 and a stairstep-shaped lower portion 64. The lower portion 64 is comprised of a first planar surface 66 positioned a predetermined distance from image plane PP, a second planar surface 68 positioned a predetermined distance from image plane PP, and a third planar surface 70 positioned a predetermined distance from image plane PP. The first surface 66 is positioned in alignment with component beam 40, the second surface 68 is positioned in alignment with component beam 42, and the third surface 70 is positioned in alignment with component beam 44.
The path length compensator prism 61 and beam splitter 26 are constructed of material having the same or nearly the same index of refraction to eliminate optical aberrations. The beam splitter 26 is typically constructed from a glass material such as BK-7. However, due to the expense of fabricating the path length compensator prism 61 from a glass material, the path length compensator prism 61 is typically molded from a plastic material such as polycarbonate. A medium 72 such as air having an index of refraction less than the index of refraction of the prism 61 interfaces with the prism surfaces 66, 68, 70.
In order for the color component beams 40, 42, 44 to pass through the path length compensator 60 and be received by the photosensor assembly 50 linear photosensor arrays 52, 54, 56, the beam splitter 26 must be positioned in fixed, oblique relationship to the path of the imaging light beam 24. As described above, the path length compensator 60 may comprise an angled, planar surface 62 upon which the beam splitter 26 may be fixedly mounted using an optical adhesive medium 80. Specifically, the beam splitter 26 comprises a planar surface 28 which is positioned directly adjacent to the path length compensator angled, planar surface 62, with an optical adhesive medium 80 disposed therebetween. To achieve adequate bonding of the surfaces 28, 62 by the optical adhesive medium 80, each of the surfaces 28, 62 must be of optical quality in terms of flatness and surface roughness.
Mounted in an optical scanner (not shown), the color imaging assembly 10 may operate at temperatures of, e.g., between -5.degree. C. and 40.degree. C. The beam splitter 26 planar surface 28 is typically a glass surface and the path length compensator 60 surface 62, which interfaces with surface 28 with an optical adhesive medium 80 disposed therebetween, is typically a plastic surface. Because of the difference in the properties of glass and plastic, any differential change in temperature of the color imaging assembly 10 causes the plastic surface 62 of the path length compensator 60 to expand or contract at a higher rate than the glass surface 28 of the beam splitter 26. More specifically, the coefficient of thermal expansion of plastic may be an order of magnitude higher than the coefficient of thermal expansion of glass. For example, the coefficient of thermal expansion of a plastic material such as polycarbonate may be, e.g., 6.6.times.10.sup.-5 /.degree.C., and the coefficient of thermal expansion of a glass material such as BK-7 may be, e.g., 7.1.times.10.sup.-6 /.degree.C. Due to the difference in the rates of expansion of these materials, the beam splitter planar surface 28 and the path length compensator surface 62 may experience varying degrees of stress because of the differential changes in temperature of the color imaging assembly 10 under standard operating conditions. This stress may become too high for the optical adhesive medium 80 disposed between the surfaces 28, 62 to adequately maintain optical contact between the surfaces 28, 62, causing the color imaging assembly 10 to fail.
This, it is an object of the present invention to provide a color imaging assembly which eliminates the need for an optical adhesive medium between a beam splitter means and a path length compensator device.
It is a further object of the present invention to provide a color imaging assembly which compensates for the differences in the coefficients of thermal expansions of a glass beam splitter means and a plastic path length compensator device.
It is also an object of the present invention to provide a color imaging assembly which includes a beam splitter means housed within a unitary path length compensator device.
It is a further object of the present invention to provide a method for producing a beam splitter means housed within a unitary path length compensator device.
It is a further object of the present invention to provide a color imaging assembly which includes a beam combiner means housed within a unitary path length compensator device.